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Question
Assertion :The net force on a closed circular current carrying loop placed in a uniform magnetic field is zero. Reason: The torque produced in a conducting circular ring is zero when it is placed in a uniform magnetic field such that the magnetic field is perpendicular to the plane of loop.
Assertion :The net force on a closed circular current carrying loop placed in a uniform magnetic field is zero. Reason: The torque produced in a conducting circular ring is zero when it is placed in a uniform magnetic field such that the magnetic field is perpendicular to the plane of loop.
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Solution
The correct option is C Assertion is correct but Reason is incorrect
The magnetic field is perpendicular to the plane of the paper. Let us consider two diametrically opposite elements as shown in the
figure.Since the direction of current is opposite, forces will cancel
each other due to →dF=i(→dl×→B)
Similarly, all forces acting on the diametrically opposite elements will cancel out in pair. The net force on the loop will be zero.
The line of action for two forces acting on dl have the same line of action.
∴ →dτ=0
Hence, net torque acting on the loop is also zero.
The reason provided does not mention about the net force.
The magnetic field is perpendicular to the plane of the paper. Let us consider two diametrically opposite elements as shown in the
figure.Since the direction of current is opposite, forces will cancel
each other due to →dF=i(→dl×→B)
Similarly, all forces acting on the diametrically opposite elements will cancel out in pair. The net force on the loop will be zero.
The line of action for two forces acting on dl have the same line of action.
∴ →dτ=0
Hence, net torque acting on the loop is also zero.
The reason provided does not mention about the net force.
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